For decades phosphorus in the form of zinc dihydrocarbyldithiophosphates (ZDDPs) has been used as extreme pressure (EP) and antiwear additives in engine oils. A problem with the use of phosphorus, however, is that it may contaminate emissions control systems catalysts and thereby reduce their effectiveness. In response to this problem, phosphorus concentration has been reduced for some SAE passenger car engine oil classifications. With the introduction of ILSAC GF-1, phosphorus levels were limited to no more than 1200 parts per million (ppm) and with GF-3 to 1000 ppm. Even at these levels of phosphorus, however, catalyst contamination is still an issue.
Various attempts have been made to reduce the phosphorus content of exhaust by reducing the contribution thereto from phosphorus in the engine lubricant. In one approach, published application US 2005/0166868 describes a method of lubricating an internal combustion engine by selecting a lubricating oil composition including a metal salt of one or more phosphorus-containing compounds represented by the formula
wherein the average total number of carbon atoms in hydrocarbyl groups R1 and R2 for the one or more phosphorus-containing compounds is at least 10.4. Although such approaches have provided a measure of success, the problem remains to provide adequate engine lubrication and at the same time further reduce phosphorus contribution to the exhaust and the corresponding risk for catalyst contamination. The present invention provides a solution to this problem.